Theoretical study of the spectroscopy and radiative transition probabilities of Si₂ from visible to infrared

High level ab initio calculations on the electronic states of a silicon dimer (Si₂) have been carried out by employing a multi-reference configuration interaction plus Davidson correction (MRCI + Q) approach with the aug-cc-pVQZ basis set. The scalar relativistic correction is taken into consideration by the second-order Douglas-Kroll-Hess approximation. In the present work, the transition properties (oscillator strength, Einstein spontaneous emission coefficient and radiative lifetime) of the singlet-singlet, triplet-triplet, and quintet-quintet transitions of Si₂ are discussed. We emphasize the triplet-triplet emission bands H³Σ-u-X³Σ-g, K³Σ-u-X³Σ-g and D³Π_u-L³Π_g which are dominant for 0-11 (18 822 cm^-1), 0-0 (30 672 cm^-1), and 0-0 (28 881 cm^-1) transitions, respectively. In addition, the strong experimentally observed b¹Π_u-d¹Σ+g band around 4184 cm^-1 corresponds to the second ¹Σ+g-b¹Π_u combination in the infrared region. The calculated oscillator strengths of the singlet-singlet transitions (f¹Π_g-e¹Σ-u, 2¹Π_g-b¹Π_u, b¹Π_u-d¹Σ+g and g¹Δ_u-a¹Δ_g) are in the order of 10^-3. From a theoretical point of view, the 0-0 sub-band for the f¹Π_g-e¹Σ-u transition, 0-7 for 2¹Π_g-b¹Π_u, 0-0 for b¹Π_u-d¹Σ+g and the 0-7 vibronic bands for the g¹Δ_u-a¹Δ_g transition may be observed experimentally. It is expected that the present results could provide theoretical support for a deeper understanding of the experimental Si₂ spectra providing further applications in astrophysics.